1. Technical Field
The present invention relates to obtaining information on texture of an object, and in particular to obtaining information on glossiness and unevenness of an object.
2. Related Art
Surfaces of objects have many different textures as well as colors. Texture of an object includes glossiness and unevenness of the object. For example, a surface of a polished metal has a smooth and glossy texture, whereas a surface of cloth or fabric has a unique uneven texture caused by the warp and woof of the cloth or fabric.
FIG. 18 illustrates the nature of reflection of light from an object. It is generally understood that when light is impinged on a surface of an object at an incident angle θ1 and reflected from the object at a reflection angle θ2, the reflection angle θ2 is equal to the incident angle θ1 (Law of Reflection). However, in reality, light is not only reflected from the surface of an object at the reflection angle θ2 but is also reflected at other angles.
This is because a reflective plane (a surface of an object) is not always flat, and has a degree of unevenness. When a reflective plane has such unevenness, the light is reflected at various angles due to the unevenness.
In the present invention, “specular reflection” means a reflection of light from a macroscopic reflective plane with a reflection angle which is substantially equal to an incident angle, and “specularly reflected light” means light thus reflected; and “diffuse reflection” means all reflections of light from the macroscopic reflective plane other than the specular reflection, and “diffusely reflected light” means light thus reflected.
In the attached drawings, a symbol Lsr is added to a light path indicating specularly reflected light; and a symbol Ldr is added to a light path indicating diffusely reflected light, where it is necessary to distinguish them.
As for the glossiness of an object, it is known to be expressed in terms of the intensity ratio of the specularly reflected component to the diffusely reflected component in light reflected from the object. For example, the ratio is relatively high for light reflected from a surface of a polished metal. This is because a polished metal surface has highly glossy texture. In contrast, the ratio is relatively low for light diffusely reflected from an object having less glossiness, such as cloth or fabric. Thus, glossiness of an object may be read by measuring the ratio of the specularly reflected light to the diffusely reflected light in light reflected from the object.
However, the intensity of light specularly reflected from an object tends to exceed the dynamic range of image-input elements of general optical image-reading devices. Accordingly, optical guiding units are designed to minimize the reception of the specularly reflected light from an object, and thereby maximize the reception of the diffusely reflected light from the object. Since the reflected light received by a general optical image-reading device contains a large amount of diffusely reflected light in this design, the device is unable to read glossiness of an object appropriately.
To read the glossiness of an object, a configuration is required such that both diffusely reflected light and specularly reflected light from the object are received, and glossiness can be obtained based on reflection components in each. For example, by illuminating an object with a light source to read an image mainly containing diffusely reflected light (a diffuse reflection image) and then illuminating the object with a light source to read an image mainly containing specularly reflected light (a specular reflection image), it is possible to generate a glossiness signal which indicates glossiness based on these image signals.
As for the unevenness on an object, it appears as shadows on the object. Shadows are more readily appeared, when the incident angle of light becomes larger. For example, as shown in FIG. 19, light hitting a convex portion of an object at an incident angle of θ11 causes a shadow in a region S1. Further, light hitting the convex portion at an incident angle of θ12 (>θ11) causes a shadow in a region S2. As shown in the figure, the region S2 is larger than the region S1. Thereby the unevenness of an object appears more pronouncedly when the incident angle becomes larger.
Accordingly, to read the unevenness of an object, a configuration is needed in which reading is performed at two different incident angles: a first incident angle and a second (larger) incident angle. When the object is illuminated at the first incident angle, the light reflected from the object expresses colors mainly based on diffuse reflection components of the object. When the object is illuminated at the second incident angle, the light reflected from the object expresses unevenness mainly based on the convexity and concavity (unevenness) of the surface of the object. Accordingly, when an image is formed based on both of these reflected lights, both the color of the object and the unevenness of the surface can be reproduced.
As shown in the partial cross-section of the image-reading device shown in FIG. 20, a light source 611 for illuminating an object O at a first incident angle θ11 and a light source 612 for illuminating the object O at a second incident angle θ12 are required.
However, having a second light source in an image-reading device as described above requires more space and leads to increased costs. Accordingly, it is desirable to provide only one light source, moving it between the position 611 and the position 612 shown in FIG. 20. In this case, too, however, the light source is required to move vertically (between the top and the bottom of the image-reading device). Image-reading devices are often required to be designed as small as possible vertically. Accordingly, the need for vertical movement of the light source as described above is a problem.
Moreover, if the unevenness of the surface of the object is very slight, it may be insufficient to read the unevenness by simply illuminating at a pre-determined incident angle. In such a case, shadows of sufficient size does not appear unless the second incident angle is further increased. It is therefore preferable to provide three or more light sources and to use these light sources according to the unevenness of the surface of the object, in order to read the unevenness more clearly. However, there still exists the problem that finding space to install three or more light sources is extremely difficult in an image-reading device which is required to be as small as possible vertically as described above.